The transmission of various types of information as digital data continues to grow in importance. Quadrature amplitude modulation (QAM) and Quadrature phase shift keying (QPSK) are increasingly seeing use as an attractive vehicle to transmit digital data.
As will be discussed in detail below, the methods and apparatus of the present invention may be used with QAM, QPSK and a variety of other types of modulated signals. Processing of these various types of modulated signals, both at the time of signal generation, e.g., at modulation time prior to signal transmission and, subsequently, e.g., upon receipt, often involves separate processing of in-phase (I) and quadrature phase (Q) signal components of the complex signal being transmitted. As a result of the separate processing of the I and Q signal components, amplitude and phase imbalances may be introduced into these signal components making it more difficult to achieve a constellation lock and properly demodulate a received signal than would otherwise by possible. The introduction of amplitude and phase imbalances is particularly prevalent where analog circuitry is used for separately processing the I and Q signal components. The use of some analog circuitry for the processing of I and Q signal components is common in many known QAM modulator and demodulator designs.
In order to reduce the effect of amplitude and phase imbalances, there is a need for methods and apparatus for reducing and/or correcting such imbalances.
For purposes of explanation, the methods and apparatus of the present invention will be explained in the context of an exemplary QAM demodulator embodiment. QAM and known QAM carrier recovery circuits will now be briefly discussed.
In essence, QAM relies on transmitting data as a sequence of two-dimensional complex symbols, i.e. with both in-phase and quadrature (I and Q) components. Each symbol, based upon the data it represents, takes on a specific pre-defined value. A set of all of the values available for transmission defines an alphabet which, when graphically plotted, typically on a two-dimensional basis, forms a constellation. The size and shape of the constellation depends upon the number of discrete values in the set and their spatial location in the constellation. In many cases the symbols in a constellation, when plotted, form a square pattern as is the case with 16 QAM signals wherein each complex symbol corresponds to one of 16 distinct values (states).
FIG. 1 illustrates a 16-QAM constellation 100. Each symbol in the constellation is denoted by an "x". In known 16 QAM the permissible nominal symbol values for both the x and y coordinates is (.+-.1, .+-.3) with the nominal squared magnitude being approximately 2, 10 and 18. When the constellation 100 is spinning, e.g., before carrier lock is achieved, the constellation appears to contain three rings corresponding to the squared symbol magnitudes 2, 10, and 18 of which only the inner most and middle rings 113, 117, respectively, are specifically shown. Note that the constellation points represents by an "x" form a square pattern with 4 constellation points falling in each one of the four different signal quadrants 1, 2, 3, 4.
To receive broadcast QAM data, a QAM receiver essentially samples and filters a received output of a communication channel and applies resulting filtered samples to a decoder (e.g. a Viterbi decoder). The decoder normally contains one or more slicers used to yield detected symbols. The data contained in these later symbols, if it contains compressed video information, is then appropriately decompressed to yield original source video data. To specifically accomplish QAM reception, a QAM demodulator within a receiver performs the functions of timing recovery, equalization and carrier recovery.
In QAM and QPSK, carrier recovery is typically performed on a decision directed basis and in the absence of a pilot tone. Carrier recovery creates a reference carrier against which in-phase and quadrature modulated components may be determined, e.g., both in terms of frequency and phase, such that the received demodulated symbols do not rotate. It is the carrier signal that is quadrature modulated by the symbols and then transmitted to a receiver. In order for carrier recovery to properly function, the amplitude and phase imbalances between the received I and Q components of a QAM signal must be relatively small so that a frequency lock may be achieved between the reference carrier and the demodulated symbols.
In some known implementations of quadrature amplitude modulation (QAM) modulators and demodulators, differences in amplitudes between the in-phase (I) and quadrature phase (Q) signal components can exist. Differences in amplitude between I and Q signal components is sometimes referred to as amplitude imbalance or unbalance. In addition to amplitude imbalance, phase imbalance may also occur between I and Q signal components. Phase imbalance occurs when the I and Q signal components are not in perfect quadrature, i.e., the signals are not 90 degrees to each other. When amplitude and/or phase imbalance occurs, a received signal will have a reduced noise threshold because the received symbols will be in the wrong place and nearer to the slicer decision thresholds of incorrect symbol boxes. Accordingly, reduced slicer performance may result from amplitude and/or phase imbalances. In addition, other receiver circuits that depend upon correct decisions, e.g., a decision directed automatic gain control circuit, carrier recovery circuits and/or equalizer update circuits, may also exhibit reduced performance in the presence of amplitude and/or phase imbalance.
FIGS. 2A and 2B illustrate the effect of amplitude imbalances on the shape of a symbol constellation. In addition, FIGS. 3A and 3B illustrate the effect of phase imbalances on the shape of a symbol constellation. Note than in FIGS. 2A, 2B, 3A and 3B, dashed lines are used to illustrate the ideal shape of the symbol constellation. In the same figures, solid lines are used to illustrate the distorted shape of the symbol constellation resulting from the particular amplitude or phase imbalance being illustrated.
In FIG. 2A, the Q component is too small relative to the ideal, e.g., sliced symbol values. In FIG. 2B the Q component is too large relative to the ideal, e.g., sliced symbol values. Note that in either case, the constellation assumes a rectangular, as opposed to a square, shape. The orientation of the rectangle is indicative of the type of amplitude correction required.
FIGS. 3A and 3B illustrate the distorting effect that phase imbalances can have on the shape of a symbol constellation. Note that the phase imbalances cause the symbol constellation's normally square shape to assume one of two possible diamond shaped patterns.
Various known systems attempt to keep amplitude and phase imbalances between real (I) and imaginary (Q) signal components small through the use of relatively accurate circuitry, e.g., in both the transmitter and receiver. This requirement for a high degree of accuracy in transmitter and demodulator system components adds to the cost of such known systems and fails to provide for any means of correcting or compensating for such imbalances when they occur.
FIG. 4 illustrates part of a known demodulator system 400. The input to the system 400 is a received complex signal representing a plurality of symbols. The input signal may be, e.g., the output of an equalizer.
The known system 400 comprises two basic loops, an inner carrier recovery loop formed by a mixer 404, a slicer 406 and a carrier recovery circuit 410 coupled together as illustrated in FIG. 4. The mixer 404 receives a complex signal including, e.g., symbols, and mixes them with the complex carrier recovery circuit output signal to generate a series of derotated symbols. When the carrier recovery circuit 410 achieves a frequency lock, the constellation represented by the symbols included in the output of the mixer 404 stop rotating allowing for proper decoding of the received symbols. In the FIG. 4 embodiment, the slicer 406 compares the values of the symbols output by the mixer 404 to a set of target or ideal values. In the case of 16 QAM, for each received symbol, a comparison is made between the received symbol value and a set of 16 possible target values. For each received symbol, the slicer 206 outputs a sliced symbol value Z.sub.SL which is the target symbol value that is closest to the received symbol value. The output of the carrier recovery circuit 410 is determined as a function of both the received and target symbol values. Since the carrier recovery output signal is a function of the decision made by the slicer 406, the signal is sometimes referred to as a decision directed carrier recovery signal.
As discussed above, the demodulator system 400 also includes an outer overall amplitude control loop. This outer loop is formed by a half-complex multiplier 402, the mixer 404, the slicer 406 and an overall amplitude gain control circuit 408. The overall amplitude gain control circuit 408 generates a decision directed gain control signal as a function of the symbols output by the mixer 404 and the sliced symbols output by the slicer 406. This may be done, as is known in the art, by comparing the received and target symbol values and generating a gain control signal as a function thereof in an attempt to adjust the input signal level so that the output of the mixer will more closely approximate the target symbol values. Since the half-complex multiplier 402 applies the same gain to the I and Q signal components of a received complex signal, it normally has no effect on the amplitude or frequency imbalances that may exist between these two signal components.
While the illustrate demodulator system works well in many applications, it does not compensate for or correct, amplitude and/or phase imbalances. Accordingly, there is a need for methods and apparatus which can detect and correct or compensate for amplitude and/or phase imbalances between I and Q components of a modulated signal.